Manganese plays a versatile and often essential role in the biochemistry of many microorganisms, plants and animals, with numerous enzymes that exploit the redox capabilities of this element. Included in this group are the manganese catalases, superoxide dismutase, ribonucleotide reductase and the oxygen evolving complex (OEC). We are evaluating new model compounds that will bring higher levels of chemical understanding to the reactivity of these manganese enzymes that metabolize dioxygen or its reduced forms. While important information has been extracted from structural modelling studies, we believe that this field has matured to an extent that reasonable questions regarding the reactivity of manganese ensembles can be proposed and that the information gleaned from these studies can suggest enzymatic experiments that further the mechanistic understanding of these systems. The three systems for which we will prepare reactivity models for are the oxygen evolving complex (OEC) that catalyzed the oxidation of two molecules of water to dioxygen, the Mn catalase that catalyzes the destruction of hydrogen peroxide to form water and dioxygen, and the manganese superoxide dismutase that disproportionates superoxide into dioxygen and hydrogen peroxide. Present understanding of the OEC suggests that dimers and tetramers in the Mn(III), Mn(IV) and possibly Mn(V) oxidation level may participate in the catalytic cycle. Therefore, we will prepare molecules in these oxidation levels that show reactivity with hydrogen peroxide or enzymic reductants such as hydroxylamine and water. For the manganese catalase we are evaluating structural and reactivity models for the dinuclear center of this enzyme by preparing Mn(II/II), Mn(II/III), Mn(III/III), Mn(III/IV) and Mn(IV/IV) dimers with variable metal separations. Finally, we are proposing a new series of ligands that will allow us to expand our effort into superoxide dismutase models. Unlike the catalase or OEC, MnSOD has been structurally characterized. Therefore we will prepare corroborative structural models and evaluate their reactivity with H02 and H202. While the primary emphasis of this proposal will be on elucidating the structure and mechanism of redox catalyzed reactions, the insight gained upon completion of these studies will result in a greater fundamental understanding of manganoenzyme structure and reactions. This should also provide a firm foundation of understanding for the evaluation of new manganoenzymes, redox and non- redox, that are likely to be discovered during the course of these studies.